1. The Field of the Invention
The present invention is related to the field of endodontistry. More particularly, the invention is related to systems and operating methods for the preparation of root canals for obturation. The systems and methods involve the use of at least instruments which are dedicated for specific purposes in the inventive methods and systems and are designed for minimal intrusion into the apical portion.
2. The Relevant Technology
To preserve a tooth with a pulp that is diseased or is potentially diseased, it is generally necessary to remove as much of the pulp material as is possible from the pulp canal of the tooth, to shape the root canal(s) without excessively weakening the root canal walls, to prevent or minimize the presence of bacteria through the use of irrigants and dressings, and lastly, to clean the walls of the root canal(s) by removing the smear layer created during instrumentation of the root canal(s). These steps are all done to prepare the root cavity for sealing or obturation which involves filling the root canal with biocompatible materials, such as gutta percha, before the pulp cavity is sealed, thereby promoting the healing and functional recovery of the tooth. This procedure is referred to as root canal therapy.
As indicated hereinabove, root canal preparation involves pulp removal, cleaning of the root canal walls and shaping of the canal walls. This is typically achieved through a guided procedure with the use of instruments which are moved either manually, mechanically or by combinations thereof. These instruments are files or bits that are configured to bore and/or cut. Mechanical instrumentation can be achieved through the use of endodontic handpieces coupled to instruments such as files. The endodontic handpieces can impart rotational motion to a file, reciprocal motion by alternately rotating a file clockwise and counterclockwise, sonic movements or ultrasonic movements.
Before endodontic therapy is begun, a preoperative x-ray image is obtained to assess the health and the pathological status of the tooth and to determine the approximate initial length of the root canal(s). Once the approximate length of the root canal(s) has been determined, an instrument can be selected for use in the root canal which has an appropriate working length.
The schematic representations shown in FIGS. 1A and 1B are similar to a typical x-ray image. As shown in FIGS. 1A and 1B, an x-ray image of teeth generally show teeth 10 with sufficient clarity to view some of the properties of roots 12 and the root canals 14 located therein, particularly the location of the radiographic apex 17. The location of the radiographic apex often does not coincide with the true apical terminus of the canal just beyond the apical foramen 16. The distance between radiographic apex 17 and a fixed reference position on the occlusal surface of a tooth is used to determine the working length of the instruments. FIG. 1B, which is an enlarged view of root 12a shown in FIG. 1A, shows the relative position of the radiographic apex designated at line 17 in relation to that of the endodontic apex and the anatomical apex designated respectively by lines 18 and 19. This condition is typical of an apex in living teeth. whereas a pathological apex can appear in a partially autolyzed state, as shown in FIG. 34C.
Preoperative or intraoperative x-ray images of a tooth requiring endodontic treatment, such as the x-ray image depicted in FIG. 1A, are obtained by lingual placement of film packets as shown in FIG. 2 at 22 which is supported by an x-ray film packet holder (not shown) and a long cone x-ray head (not shown) located outside of the cheek. Although, x-ray images obtained as shown in FIG. 2 from a buccal-lingual x-ray projection are generally useful for determining the overall characteristics of a tooth, the approximate initial length of the root canal(s), and the working length for a file, such images provide only limited information regarding the overall anatomy of the root canal.
The information is limited because only one dimension of the overall anatomy of the pulp cavity can be viewed in vivo. In the standard buccal-lingual projection such images show only a linear profile of the root canal and cannot show a tridimensional view of a tooth and its root canal(s). Although, it would be very helpful to view a tooth from a position between the teeth or from the interproximal space such a mesial-distal view cannot be clearly produced when the tooth is still positioned in a patient's mouth. Since information is needed of all three dimensions in order to correctly understand the overall anatomy of the root canal and yet only two-dimensional images of a tooth can be obtained, x-ray images are sometimes relied on to reach incorrect conclusions regarding the anatomy of the root canal. More particularly, if not properly evaluated, x-ray images can be misleading as to the actual length of the root canal and the position of the foramen or foramina.
The difficulties encountered by an endodontist in assessing the overall anatomy of teeth from just the x-ray images obtained from buccal-lingual x-ray projections can be clearly identified with reference to FIGS. 3-6. FIGS. 3A-6A are longitudinal cross-sectional schematic views of extracted teeth taken from the front or back of the respective tooth which correspond with typical images obtained from buccal-lingual x-ray projections FIGS. 3B-6B are longitudinal cross-sectional schematic views of the same extracted teeth shown respectively in FIGS. 3A-6A taken from the mesial-distal or side view that cannot be obtained or seen while the teeth are still positioned in a patient's mouth.
Note that by varying the angle of incidence of the x-ray beam mesiodistally and distomesially additional x-ray images can be made which provide some additional information about the anatomy of the tooth. However, we can never obtain a three-dimensional image!
FIGS. 3-4 illustrate that in order to properly prepare a root canal it is necessary for practitioners to rely heavily on their experience, knowledge acquired through a study of typical anatomical structures, and on their visually acquired experience with longitudinal and transverse dental cross-sections at various heights. FIG. 3A depicts a lower premolar 30 from the buccal-lingual view of the tooth which shows root 32 and a root canal 34 therein that appears to be rather narrow and to have a relatively uniform perimeter along its length. FIG. 3B, however, shows that when seen from the mesial-distal view, the root canal is initially fairly wide over more than half its length, and then tapers significantly before eaching the apical foramen 36. Comparing FIG. 3A with FIG. 3B clearly shows that when limited to knowledge derived from an x-ray corresponding to the image shown in FIG. 3A, the practitioner may not be able to accurately assess the anatomical structure of the root anatomy. Additionally, FIG. 3C shows that instead of an apical foramen there may be double, triple or quadruple foramina as indicated in research performed by the applicant and by others. The triple foramina 36a, 36b and 36c shown in FIG. 3C may not be detectable when viewed only from the buccal-lingual view shown in FIG. 3A.
FIG. 4B depicts an upper premolar 40 with roots 42a and 42b and root canals 44a and 44b located therein. Comparing FIG. 4A and FIG. 4B reveals a problem which is similar to the problem revealed by comparing FIG. 3A with FIG. 3B. More particularly, by comparing FIG. 3A with FIG. 3B or FIG. 4A with FIG. 4B, it is easily understood that the practitioner may not be able to accurately assess the anatomical structure of the root anatomy when limited to knowledge derived from an x-ray image. Since the configuration of pulp chamber 48 may be difficult to accurately and fully ascertain from only an x-ray image, a practitioners also relies, as indicated hereinabove, on accumulated experience, knowledge of dental anatomy, and knowledge of typical anatomical structures.
The potential inaccuracy of a conclusion derived from information obtained from an x-ray image is further illustrated by comparing FIG. 5A and FIG. 5B and also FIG. 6A with FIG. 6B. More particularly, as discussed hereinbelow, FIGS. 5-6 show that practitioners encounter anatomies with widely varying aberrations and intercommunications of root canals which may not be apparent to the practitioner from the limited information derived from x-ray images.
FIG. 5A depicts a mandibular or lower incisor 50 from the buccal-lingual view of the tooth which shows root 52 and root canal 54. FIG. 5B depicts the same lower incisor 50 from the mesial-distal view of the tooth. The mesial-distal view shown in FIG. 5B clearly reveals that root canal 54 branches and then rejoins to have a single foramen 56. Root canal morphological variations, such as that shown in FIG. 5B, may not be detectable by a practitioner who relies solely on a preoperative or intraoperative x-ray image, such as the image of lower incisor 50 shown in FIG. 5A.
Similarly, a root canal may branch without merging so as to yield multiple foramina, such as the root canal of a lower first molar 60 shown in FIGS. 6A-6B. Again, the buccal-lingual view, as shown in FIG. 6A, provides inadequate information when compared with the depiction taken from the mesial-distal view of mesial root 62b in FIG. 6B. FIG. 6B reveals that branches 64a and 64b do not merge and accordingly have two foramina 66a and 66b.
In addition to the morphological variations in anatomy as discussed above, consideration should also be given to the substantially different perimetrical configurations of root canals, as revealed by various dental cross-sections shown in FIG. 7 and FIG. 8. Additionally, the shape of root canal perimeters varies not only between different types of teeth as shown in FIG. 7 and FIG. 8 but also along the length of a single root canal of a tooth as is illustrated in FIGS. 9A-9B.
FIG. 7 shows a classification system devised by A. Latrou which divides the perimetrical anatomies of root canals into those that have primarily a tubular morphology and those that have primarily a laminar morphology. Examples of root canals with tubular perimetrical anatomies are shown at 70, 71, and 72 which are respectively primarily oval, round and triangular. The laminar perimetrical anatomies include root canals with essentially slit-like configurations such as those shown at 73, 74, and 75 which are respectively primarily straight, semi-lunar shaped, and figure eight shaped due to the vestibular and lingual bulges. The laminar shape is more common than the tubular type root canals.
FIG. 8 shows cross-sectional views of the middle third of different teeth 80a-80l that have been extracted and then cut along a transverse cross-section of the tooth to show root canals 82a-82l as well as corresponding pulp chambers 84 and floors or cervical aspects 86. Conventional file instruments 88 are also shown inserted into root canals 82. From this view, which can only be seen in vitro, it is evident that a certain degree of variation occurs in the perimetrical anatomy of the pulp cavity of teeth. The practitioner may at first be unaware of such variation; however, the practitioner must keep in mind the possibility of such variation while working with instruments in the root canal so that all of the canal walls will be treated and the irregularities caused by greater parietal thicknesses can be removed without unduly weakening the walls.
FIGS. 9A depicts a maxillary right upper first molar 90 with dashed cutting lines included to identify the division of the tooth into transverse cross-sections for segmentation as shown in FIG. 9B. FIG. 9B displays roots, 92a, 92b and 92c of molar 90 as cut into four respective segments, 100-103, to clearly show the variations of root canals 94a, 94b and 94c. Also displayed in FIG. 9B are segments 104 and 105, which respectively contain the pulp chamber 98 and its floor. A comparison of the perimeters of root canals 94a, 94b and 94c starting at segment 104 as each root canal tapers to its respective apices 96a, 96b and 96c, clearly shows that the perimeter anatomy varies and transitions in configuration along the entire length of each root canal. So not only must a practitioner deal with root canals having different shapes as discussed in reference to FIG. 7 and FIG. 8, but the practitioner must also utilize an instrument in a root canal with a perimetrical or circumferential anatomy that varies depending on the height at which the observation is made.
From the discussion above, it is apparent that when a practitioner views a preoperative or interoperative x-ray image of a tooth, the practitioner can only guess about the actual anatomy of the pulp cavity and the root canal(s) of the tooth. While the practitioner may be able to confirm that a root canal has been cleaned along the length of the pulp chamber from the coronal portion to the apex of the root, the length that has been contacted or abraded by the file may only be a portion of the root canal system.
Since it is impossible to obtain a mesial-distal view of the root canal or to view the perimetrical anatomy on different points along the length of the root canal, the practitioner is prevented from obtaining a proper preliminary understanding of the overall root canal anatomy in order to assess the necessary relationship between the canal walls and the instrument inserted in the root canal. Accordingly, as shown in FIG. 10A and FIG. 10B, when a file instrument such as instrument 114 is inserted as far as the apex into a root canal such as canal 112a of tooth 110a or canal 112b of tooth 110b and then rotated, significant portions are not cleaned.
The inability to clean all surfaces of a root canal by merely inserting and rotating a file instrument in a root canal is further illustrated by FIG. 8. FIG. 8 depicts the position of file instrument 88 in transverse cross-sectional views of root canals after file instrument 88 has been inserted to the apex of each respective root canal. FIG. 8 clearly shows that simply drilling from one position into the root canal will often miss large sections of the perimeter of the root canal, thereby leaving portions of live, diseased or necrotic pulp material undisturbed. If the operator is unable to apply the instrument to every segment of the perimeter of the canal, the undisturbed pulp material may ultimately cause undue pain, lengthy healing times or even cause the procedure to fail. Solvents or irrigants such as sodium hypochlorite may be used to flrther clean the root canal.
The next step is neutralization or obturation of the root canal which involves coating or filling the root canal with a plastic obturation material such as heated gutta percha. The object of obturation is to prevent the ingress of bacteria or tissue fluids which might act as a culture medium for any bacteria remaining within the root canal system by sealing the system. In order to reach the recesses with the filling material that cannot be treated with the instruments, vertical pressure must be applied with a plugger; however, there is never any assurance that all of the necrotic residue has been coated. There is also a risk that such techniques will cause infected material to be pressed beyond the apex. Such an extrusion of infected material beyond the apex is very undesirable as it may contain polymicrobial loads or charges that may produce damaging bacteremia or cause chronic inflammations of the apical and periapical tissues.
Based on all of the foregoing observations, it can be concluded that inadequate attention is given to understanding the dental chambers on a three-dimensional basis, the varying configurations of the perimeter of the root canal(s), the diameter of the canals, and the thickness of the walls. Note that research is still needed to investigate and catalog the thickness of dental walls in order to increase understanding amongst practitioners.
The inability to fully identify the anatomy of the pulp cavity restricts the ability of the practitioner to confidently conclude that the procedure has been successful. Although problems may result from having incomplete information regarding the anatomy of a particular root canal, many practitioners using conventional methods and instruments are not overly concerned with completely cleaning the entire root canal since their failure rate is not at an unsatisfactory level. While these conventional methods and instruments may result in satisfactory failure rates, it would be very beneficial to still lower the failure rate and to better preserve the integrity of teeth.
As discussed hereinbelow, most of the methods and instruments that have been and continue to be employed and produced are relatively arbitrary with regard to root canal anatomy. To compensate for the limited understanding of the inability to contact all root surfaces and the lack of knowledge of the actual anatomy of the root canal, many working methods have been devised, which in turn have prompted the creation of a multitude of instruments of varying diameters and sizes.
With regard to operating procedures, there are two basic methods from which all of the canal-preparation techniques can be derived. These methods have been interpreted by various authors in an operational context and also in terms of the instrumentation. The primary conventional systems and methods for removing pulp material from the root canal of a tooth are the apico-coronal (step-back) technique and the corono-apical (crown-down) technique. Although these conventional cleaning techniques both rely generally on sequential increases in the diameter of instruments inserted into the root canal, the step-back technique involves cleaning the root canal from the apex toward the crown while the crown-down technique involves cleaning the root canal from the crown down to the apex. Each has its own unique benefits and disadvantages which are discussed hereinbelow.
The step-back technique involves the use of various sets of file instruments which are sequentially inserted into a root canal after the root canal has been exposed by removing the roof of the pulp chamber as depicted in FIG. 11A and FIG. 11B. More particularly, before pulp material 160 can be removed in accordance with the step-back technique, an instrument, such as instrument 120 shown with bur 122 in FIG. 11A and FIG. 11B, is utilized to remove the overhanging portions of enamel 152 and dentin 154 in order to provide access into the pulp chamber 156. FIG. 12 depicts a set of step-back file instruments with each file instrument 130 comprising a handle 132 connected to a file 134 or a shaft with tines or an abrading portion. Each file has a tip 136 opposite a top end 138 where file 134 joins handle 132. As viewed in FIG. 12 from left to right, the diameter at top end 138 of each file increases progressively from the smallest to the largest such that the diameter of 138a is less than the diameter of 138b. The diameter of each successive file at tip end 136 is also successively larger. Accordingly, the taper of each file remains essentially the same even though each file is progressively larger that the preceding file.
In the step-back technique, the apical portion of the tooth is prepared first and then the remainder of the canal is flared from apex to crown. This process essentially involves inserting a series of progressively larger files into the apex of the root canal and rotating each file and/or moving the file up and down in a longitudinal motion until a file can be inserted that is considered to be a suitable standard size for completing the process or that meets some resistance to rotation. The rest of the canal is then flared by sequentially using each file in the set, as shown in FIG. 12, with each file being larger than the preceding file and by alternately advancing and then withdrawing each instrument.
FIG. 13A depicts a molar 150 being prepared by the step-back technique after the removal of enamel 152 and dentin 154 that extend into pulp chamber 156, and after the first stage of the step-back technique has been completed. The first stage of the step-back technique involves the insertion of a file into pulp chamber 156 and into root canal 158a in order to remove material 160 in the lower portion of the canal above the apex or apical end 162a. After the portion above apex 162a is cleaned, each file shown in FIG. 12 is sequentially inserted downward toward apical end 162a of root canal 158a, starting with file instrument 130a as shown in FIG. 13A. As a result of this technique, the diameter of the area being contacted at the apical portion is increasingly larger.
FIG. 13B is a cross-sectional view taken along cutting line 13B--13B in FIG. 13A of tooth 150 during cleaning of root canal 158a with file instrument 130a in the step-back technique. Insertion of the files of the other file instruments 130b and 130c may further clean out material 160 because each file has an increasingly larger diameter. With each increase in diameter, the rigidity increases and the flexibility of the files decreases. As a result, it becomes increasingly difficult for the files to adjust to or to follow the contours of the perimeter surfaces of the root canal. This reduced flexibility also increases the likelihood that the files will fail to contact some portions while removing too much of the surrounding dentin 154 in some areas through excessive abrasion and resulting in overthinning of the walls.
Note that the views depicted in FIGS. 13A and 13B depict the problem previously discussed with regard to the difficulty in assessing the actual root canal anatomy in vivo. When viewed in FIG. 13A, it appears that the root canal has been cleaned; however, FIG. 13B shows that a significant portion of material 160 remains. Accordingly, when the root canal is viewed in an x-ray photograph which is the same view shown in FIG. 13A, a practitioner may mistakenly believe that the tooth has been adequately cleaned. This mistaken belief may be further incorrectly relied on as the root canal is widened by the insertion of the larger files which gives an impression of complete cleaning. There is resultingly some possibility for failure of the root canal therapy due to incompleteness.
Not only is the completeness effected by the use of a set of files wherein each file is more rigid than the preceding file but the ability to safely move the file within the canal is also limited. More particularly, the increasing rigidity results in decreased ability to negotiate the curves in the canal. Significant problems that can result from inserting increasingly rigid files and also from initially inserting a file all the way down to the apex includes laceration and transportation of the apical foramen, as well as misdirection and perforation of the wall. As shown in FIG. 14A, after tooth 170 was prepared by removal of portions of the enamel 172 and dentin 174, file 132 was inserted into root canal 176 and perforated apex 178. Perforating the apex can also result from an error in estimating the length of a root canal, by failure of a stop such as stop 140 to remain at a predetermined position or by failure to observe the calibration or graduation hatch markings on the file, which can be used instead of a stop to designate the length.
The apex can be perforated by extrusion of the infected material 180 through the apex due to the force exerted by the file on the material as the file is pushed downward to reach the apex. As a result, the periapical region can be invaded and contaminated. The potential for extruding infected material through the apical foramen of a necrotic tooth during the initial insertion of a file instrument all the way down to the apex is a particular disadvantage of the step-back technique. Another disadvantage is that the procedure has identical steps for working in either necrotic or vital root canals. In addition to exposing the tissue surrounding the tooth to the infected material, apical perforations may allow irrigants, amalgam filling or obturating material to flow out of the apex. Such apical perforations, as as well perforations, may delay tooth healing and may compromise the outcome of the therapy.
Perforations can also occur due to a failure to maintain a proper working length of the instrument during the procedure. As the canal is widened, curvatures are straightened thereby decreasing the required working length needed for the instrument to work. Accordingly, the rubber stop 140 must be adjusted, thereby continually providing an opportunity for the instruments to become contaminated by bacteria. To properly determine the appropriate working length, many radiographs must be taken throughout the operation as the canal is continuously being modified, which alters the length. The time required to obtain the x-ray photographs or images and to adjust the working length of the instruments by repositioning the stops can result in a lengthy process. The step-back technique is also time intensive because a large number of instruments are required to complete the root canal therapy.
As shown in FIG. 14B, another problem is the formation of ledges such as ledge 182. Ledges can occur when a practitioner attempts to insert a file such as file 134 as far as the apex and the file is too inflexible to properly curve with the root canal or move around a protrusion. When a file is too inflexible to curve or flex as needed and is halted prematurely, the downward pressure exerted on the file, in conjunction with the tendency of the file to straighten itself, causes the tip of the file to dig into the side of the root canal and form a ledge. Such ledges are difficult to bypass; and if the ledge occurs very close to the apex, the ledge may give the practitioner the mistaken impression that the apex has been reached.
The crown-down technique was developed for several reasons. It was desired to shape the canal "conically" so as to keep the diameter of the foramen as straight as possible. The crown-down technique was also developed to prevent the discharge of septic material or obturation material from the apex after the initial canal-preparation step and to prevent subsequent vertical condensation due to the vertical pressure used to obturate the canals with heated gutta-percha. Additionally, the crown-down technique was intended to reduce the number of instruments utilized compared with the step-back technique. However, as discussed hereinbelow, significant potential problems may inherently result from use of the crown-down technique.
The crown-down technique generally involves the use of a set of file instruments wherein each file in the set of file instruments has a progressively different diameter at the top of the cutting portion of the file, i.e., the point where the file becomes smooth and no longer has cutting capabilities. The smooth portion may have a constant diameter. The diameter at the top of the cutting portion of each file may be either constant or graduated for the entire set of instruments such that the top of the cutting portion of each file is progressively larger tan that of the preceding file. As a result of this configuration, the taper of each file is larger than the preceding file in the set. By using such files of increasingly larger diameters, the area that is initially and subsequently abraded, as work proceeds toward the apex, will always be primarily at the top portion of the root canal.
Gradual progressive conicity or taper, and the constant diameter of the tip (characteristics which, paradoxically, have been inflated, despite the prior teachings of the present applicant in earlier patents, such as Italian patents No. 1,199,941 and No. 1,169,326, and U.S. Pat. No. 4,971,556) are characteristics which are now so standard among the multitude of crown-down instruments made of nickel/titanium that have been introduced onto the market, that competitors have shifted toward other features of the instruments. For example, increasing value is being attached to the so-called "overall" originality of an operating procedure that uses so-called "dedicated" instrumentation to solve the multiple problems associated with root canal preparation in terms of ergonomics, operational safety, the time and cost of the procedure, and the likelihood of success.
One example of the operational deficiency of the crown-down method lies in its association with instruments made of nickel/titanium. Based on the greater flexibility of files formed from nickel/titanium compared with files formed from steel, proponents of the crown-down method in conjunction with nickel/titanium files assert that such files can better follow the curvatures of a root canal. Additionally, it has been asserted that such files are more likely to stay in the center of the root canal, thereby decreasing the likelihood of ledging or perforating the root canal walls. As set forth hereinbelow in greater detail, each material has its own unique advantages and disadvantages.
The ability of a nickel/titanium file to stay in the center is not necessarily desirable, in view of the morphology and perimetrical variety of root canals, and particularly the variety in the upper two-thirds of laminar root canals. In fact when rotation is imparted to an instrument that stays in the center of the canal, the file instrument works simultaneously and indiscriminately on all of the walls within reach of the file. Since root canal walls do not have equal thicknesses in all directions and at all different points along a root canal, some walls can be overthinned or perforated, while other walls remain untouched.
Moreover, because nickel/titanium files are more flexible than steel files, they tend to follow the path of least resistance and therefore cannot be used, in the same way as steel files, to be applied actively and intentionally by the operator. As a result, even when the operator knows the thickness of a particular portion, such as an interference or obstruction which the operator desires to rectify or straighten, the operator lacks the freedom to aggressively drive the file as needed and clean the portions that are difficult to reach. Accordingly, when a nickel/titanium file is used to clean a non-cylindrically shaped root canal, the file moves only at the center of the canal and/or the area of least resistance and fails to remove all of the necrotic tissue.
FIGS. 15A, 15B, 15C, 15D and 15E depict transverse cross-sections of tooth 190 that has been cleaned in a manner that has resulted in either overthinning of root canal walls, perforation of a root canal wall, excessively weakening of the walls of the tooth or a failure to fully contact all of the canal walls. These problems can be easily caused by the passive, self-guiding use of nickel/titanium files with progressively larger tapers in the transition from the first instrument to the next one in the set. These problems can also be caused by the increasing rigidity, in accordance with the crown-down technique, which prevents the files from being laterally moved to enable the files to clean the entire perimeter of the root canal. The cross-sections shown in FIGS. 15A-E may be considered independently from each other as being cross-sections from different teeth or from a single tooth such that FIG. 15A shows two roots 192a and 192b of a tooth 190 while FIGS. 15B-15E show root canal 194a as the root canal tapers to the apex.
FIG. 15A depicts the overthinning that can occur to the furcation walls of root canals 194a and 194b near the bifurcation as a result from the indiscriminate thinning of the distal walls of the root canals by maintaining a file instrument in a central location during working rotation. The resulting boreholes are shown at 196a and 196b while the outlines of root canals 194a and 194b before cleaning are shown in phantom lines. Such overthinning and potential furcal perforation can have devastating results. The inability to adequately direct a file used in accordance with the crown-down technique based on the practitioner's knowledge of the relative thicknesses of the portions of canal walls is a significant disadvantage of the technique.
FIG. 15B shows a lateral perforation that has occurred when a hole was made through dentin 198 and cementun 197 during the cleaning of root canal 194a. The lateral perforation resulting from the formation of borehole 196a may be obscured from the x-ray due to concavities or curvatures in the root canal. The practitioner may then mistakenly conclude that the root canal has been successfully cleaned without realizing that there is a perforation.
In FIG. 15C, the segment shown of root canal 194a was overly thinned during the cleaning of root canal 194a as borehole 196a is shown extending through dentin 198 and into the cementum 197. As mentioned in reference to FIG. 15B, the formation of borehole 196a may be obscured from the x-ray view. As a result, the practitioner may not realize that the borehole extends into the cementum and may therefore mistakenly conclude that the root canal treatment has been successful. Infective bacteria that remained in the root canal, perhaps in the portions that were not contacted with the files, as well as toxins produced by the bacteria may then permeate through the cementum and cause infection or other complications.
FIG. 15D provides an example of a cross-section of a laminar-type root canal cleaned by the crown-down technique which may result in successful root canal therapy since the instrumentation has not resulted in a perforation and the cementum 197 has not been exposed. Although, problems such as perforations or overthinning have been avoided, FIG. 15D shows that large portions of root canal 194a remain untouched despite the change in morphology through the formation of large borehole 196a. Note that the change in the morphology of the canal shown in FIG. 15D resulting from crown-down technique instrumentation occurs due to drilling in a passive, circular manner with instruments having gradual and progressive tapers. The failure to contact significant portions of a root canal while forming a large borehole in a root canal as shown in FIG. 15B-D is a very typical result of the crown-down technique since most root canals can be characterized as a laminar-type root canal.
It would be preferable to avoid the risk posed by failing to contact significant portions of the root canal as shown in FIG. 15D. Since the practitioner is prevented from removing and cleaning essentially all pulp material, the practitioner cannot be assured of the reliability of the treatment. Additionally, the practitioner may not suspect that the working instruments have failed to contact every segment of the root canal as use of a set of files with increasingly greater tapers can contribute to a potentially incorrect conclusion that cleaning by such a conventional process has resulted in removing all material from root canal 194a. Further, the x-ray view of tooth 190, as with the step-back technique shown in progress in FIGS. 13A and 13B, would give the impression that the root canal had been cleaned. It should also be remembered that while rotation of a set of passively actuated files, with increasingly greater tapers, in the center of the canal, in accordance with the crown-down technique, may yield a configuration as shown in FIG. 15D and result in successful root canal therapy, there is a significant hazard, as shown in relation to the FIGS. 15A-C, due to the passivity of the instruments when linked to canal diameters and wall thicknesses that are still statistically unknown.
As in the configuration shown in FIG. 15D, the configuration shown in FIG. 15E may also result in successful root canal therapy--but only for canals of the wholly tubular type. Although, borehole 196a does not extend through dentin 198 and into the cementum 197, the diameter of the preparation or borehole 196a is nevertheless significantly larger than that of the original root canal was as shown by the phantom lines at 194a. The excessive thinning of the dental wall may resultingly significantly weaken the resistance of the walls to the stress of chewing, and may also cause a fracture of the root.
From the above discussion in relation to FIGS. 15A-E, it is clear that the actual morphology of the canals is not sufficiently considered when using this method and that the use of files with increasingly larger tapers limits the range of motion of the files. More specifically, due to the use of files with successively larger tapers which therefore are increasingly rigid, each file, if actuated passively, is primarily limited to being rotated without substantial lateral movements guided by the operator. Since the majority of files are of the laminar type, this limitation poses a significant problem. Without the ability to laterally move the files, it is not possible to make contact with every segment of the perimeter of the canal and some portions may receive too much contact.
In any event, if the files are rotated passively in a laminar canal or a canal which has a laminar-type anatomy for the first two-thirds of the canal, the result is a circular opening whose diameter corresponds to that of the file that was used. The file typically stays in the center of the canal during rotation, such that the tip of each file acts like a fulcrum and "ideally" stays in the same position as a rotation point. Since each successive file can move less laterally, each file simply makes a bigger borehole than the preceding file. Accordingly, the files cannot clean a root canal without significantly altering the original anatomy by leaving a footprint or borehole corresponding to the configuration of the instruments used. More specifically, the result is a footprint or borehole with a perimeter that corresponds to the perimeter of the biggest file that extends well beyond the original anatomy of the root canal and yet in most instances does not adequately clean significant portions of the root canal.
As discussed above, the flexibility of the files used in the crown-down technique, which are typically formed from nickel/titanium, prevents the files from being successfully urged against the perimeter or against the various surface features of the root canal. As also discussed above, the flexibility of the files also increases the tendency of the files to remain in the center or at the location where less resistance to movement is encountered. Accordingly, the flexibility of the files also contributes to the configuration of borehole 196a, which substantially deviates from the original anatomy of the root canal 194a.
There are also other disadvantages to the use of nickel/titanium files. The flexibility of nickel/titanium files increases the likelihood that the file may bend and be deformed upon encountering a hard substance. Since nickel/titanium files are more fragile and more flexible than stainless steel files, the nickel/titanium files can break more easily and unexpectedly than steel files. When a nickel/titanium file instrument is used with a large file diameter the flexibility decreases to the point of being as rigid as stainless steel and yet breaks more easily. More particularly, beyond a certain diameter, the upper halves of larger diameter files are still as rigid as that of steel files while the flexible lower halves of nickel/titanium file instruments are more prone to break.
Additionally, rotation of a file in a canal that has a laminar upper two-thirds exposes the tip of the file to the risk of breaking when the tip of the file is embedded or stuck in a canal whose diameter is smaller than its own diameter! To avoid breaking the tip when it is embedded or stuck in a canal whose diameter is smaller than the diameter of the tip, operators who use nickel/titanium files are advised to employ catheterization in order to obtain a prophylactic widening of the canal, using a series of instruments with increasingly larger tip diameters.
Another disadvantage of nickel/titanium files is that nickel embodied in the alloy may potentially result in an allergic reaction. Further, nickel/titanium files costs about four times as much as steel files and yet nickel/titanium files generally wear out faster than steel files. Nickel/titanium files wear out so quickly that some manufacturers mark their products as being intended for single use only.
Moreover, the crown-down instruments currently available on the market, almost all of which are made of nickel/titanium, in some respects violently conflict with the use of the crown-down method, because, paradoxically, these instruments are smooth in areas where the method requires that they first perform a cutting action. The reason for this deficiency lies in the length of the abrading portion of the instruments, which portion is only 16 mm long and which extends into a smooth portion leading to the handle, onto which rubber stops are affixed, or into which millimeter-based calibration marks are engraved in order to allow visual control of the working depth of the instrument in the canal. For example, note in FIGS. 13A and 13B that when file 134a is inserted into root canal 162a that the abrading portion 138a is not long enough to contact the dentinal shelves 166, instead the upper portion ofthe file is smooth shank portion 136a. Since the length ofthe root canal often exceeds the standard 16 mm length abrading portion of conventional instruments, see Table 1 hereinbelow, we may reasonably ask how an instrument which, according to the crown-down method, is supposed to prepare a canal starting at its coronal third, can perform this task if its coronal segment is smooth!
TABLE 1 Average Root Canal Lengths Tooth Upper Lower Central 23 mm 20.5 mm Lateral 22 mm 21 mm Canine 26.5 mm 25.5 mm FirstPremolar 20.5 mm 20.5 mm Second Premolar 21.5 mm 22 mm First Molar 20.5 mm 2l mm Second Molar 20 mm 20 mm
Obviously, from a review of the average root canal lengths in Table 1, significant segments of a root canal cannot be abraded by standard 16 mm long abrading portions of conventional files and are contacted only by the smooth portion of the files.
Although, the crown-down technique typically enables a practitioner to more efficiently clean a root canal than the step-back technique, they both require the practitioner to utilize many different instruments. The need to frequently change the cleaning instrument results in significant time requirements for cleaning a root canal. However, careful instrumentation in accordance with either tedious time consuming method does not avoid the problems set forth above in relation to apical perforation, wall perforation, overthinning or failure to clean all of the wall surfaces.
Based on the foregoing observations, methods and systems are needed in the endodontic arts which enable a dental practitioner to remove and clean essentially all pulp material in a root canal requiring root canal therapy.
It would also be an advancement in the endodontic arts to provide methods and systems that are based on the three-dimensional reality of teeth and do not relate solely to buccolingual x-ray views, thereby enabling a practitioner to remove and clean pulp material in a root canal without compromising the strength of the walls and the apical anatomy.
It would also be a beneficial development in the endodontic arts to provide methods and systems which encourage perimetrical contact of the instruments with the canal walls.
Additionally, it would be an advancement in the endodontic arts to provide methods and systems that enable a practitioner to remove and clean pulp material in a root canal in a manner that is less likely to result in failure due to bacterial contamination, overly thinning the root canal, perforations or due to infected material being pushed beyond the root from the coronal aspects of canals.
Finally, it would also constitute progress in the endodontic arts to provide methods and systems which yield a predictable success rate, minimal risk of breaking an instrument, lower costs, and an abbreviated operating time or an operating time that is at least as efficient as conventional techniques.